Within and beyond the stringent response-RSH and (p)ppGpp in plants

Abstract

Main conclusionPlant RSH proteins are able to synthetize and/or hydrolyze unusual nucleotides called (p)ppGpp or alarmones. These molecules regulate nuclear and chloroplast transcription, chloroplast translation and plant development and stress response.Homologs of bacterial RelA/SpoT proteins, designated RSH, and products of their activity, (p)ppGpp—guanosine tetra—and pentaphosphates, have been found in algae and higher plants. (p)ppGpp were first identified in bacteria as the effectors of the stringent response, a mechanism that orchestrates pleiotropic adaptations to nutritional deprivation and various stress conditions. (p)ppGpp accumulation in bacteria decreases transcription—with exception to genes that help to withstand or overcome current stressful situations, which are upregulated—and translation as well as DNA replication and eventually reduces metabolism and growth but promotes adaptive responses. In plants, RSH are nuclei-encoded and function in chloroplasts, where alarmones are produced and decrease transcription, translation, hormone, lipid and metabolites accumulation and affect photosynthetic efficiency and eventually plant growth and development. During senescence, alarmones coordinate nutrient remobilization and relocation from vegetative tissues into seeds. Despite the high conservancy of RSH protein domains among bacteria and plants as well as the bacterial origin of plant chloroplasts, in plants, unlike in bacteria, (p)ppGpp promote chloroplast DNA replication and division. Next, (p)ppGpp may also perform their functions in cytoplasm, where they would promote plant growth inhibition. Furthermore, (p)ppGpp accumulation also affects nuclear gene expression, i.a., decreases the level of Arabidopsis defense gene transcripts, and promotes plants susceptibility towards Turnip mosaic virus. In this review, we summarize recent findings that show the importance of RSH and (p)ppGpp in plant growth and development, and open an area of research aiming to understand the function of plant RSH in response to stress.

A riboswitch gives rise to multi-generational phenotypic heterogeneity in an auxotrophic bacterium

“…These tRNAs bind to the riboswitch 41 , where they trigger formation of the anti-terminator complex that prevents transcriptional attenuation by the terminator hairpin 40,46 . In addition, tRNAs trigger the stringent response 47,48 , leading to a physiological state in which cells presumably stay locked 21,49,50 for several generations. Ultimately, the increased expression of the highaffinity transporter increases the rate of methionine uptake and, thereby, supports growth in the second subpopulation of cells (GFP+).…”

Identity and functions of inorganic and inositol polyphosphates in plants

“…Enzymes catalyzing the synthesis and breakdown of pppGpp in plants (van der Biezen et al, 2000) have been shown to function in the chloroplast, where pppGpp accumulates under stress conditions (Chen et al, 2014). It has been speculated that PPX could cleave pppGpp, regulating stress responses (Boniecka et al, 2017). However, the enzymatic The number of proteins with primary and secondary structure homology to inorganic polyphosphate (polyP)-metabolizing enzymes is listed.…”

The conserved theme of ribosome hibernation: from bacteria to chloroplasts of plants

“…However, PSRP1 expression seems to be directly controlled by the circadian clock, as PSRP1 transcripts peak directly after onset of night in synchronized diurnally grown Chlamydomonas reinhardtii cells (Zones et al, 2015) and also show clockdependence in higher plants (Graf et al, 2017a,b). Interestingly, components of the stringent response seem also conserved in plastids (reviewed in Boniecka et al, 2017;Field, 2018), suggesting that chloroplast metabolism is also regulated by the stringent response. Homologues of RelA have also been found in plants and seem to localize predominantly to chloroplasts (reviewed in Boniecka et al, 2017).…”